microwave-assisted synthesis and sintering of nzp compounds

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Microwave-Assisted Synthesis and Sintering of NZP Compounds Balasubramaniam Vaidhyanathan, Dinesh K. Agrawal,* ,‡ and Rustum Roy Microwave Processing and Engineering Center, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 We have identified the high microwave susceptibility of the sodium di-hydrogen phosphate monohydrate (NHPM), NH 2 PO 4 H 2 O. This acid phosphate of sodium can be heated to >900°C when exposed to 2.45 GHz microwave radiation. Using NHPM and microwave energy, a novel single-step synthesis of many important crystalline NZP compounds has been accomplished in a very short time. Interestingly, the combination of microwaves and nonstoichiometric oxide pre- cursors for the preparation of NZP materials is found to reduce the reaction temperatures and enhance the reaction kinetics further. The microwave synthesis method is found to be simple and fast, minimizing the loss of volatile chemical species from the reaction. A microwave-assisted procedure for the rapid sintering of NZP compounds has also been demon- strated. Densification (>97%) has been achieved in <30 min at sintering temperature much lower than normally required in conventional processes. Finer microstructure and better den- sification are the main advantages of the microwave sintering in this study. I. Introduction A PARENT compound of a very large family of important and interesting inorganic phosphates, NaZr 2 (PO 4 ) 3 , commonly known as NZP, 1,2 has a special open framework structure 3 with corner-linked ZrO 6 octahedra and PO 4 tetrahedra. 4 The covalent network skeleton NZP is remarkably stable toward temperature and chemical substitution. 5,6 Compounds based on NZP structure have found application as superionic conductors, 5 candidates for ceramic nuclear waste form, 2,7 catalysts for NO x reduction, and are suitable for low thermal expansion applications. 8 The conventional solid-state synthesis methods presently available for the prepara- tion of NZP compounds are complex and time-consuming. 9 –11 Typical solid-state synthesis method employs a high calcination temperature of 1100 o –1300°C and long soaking times in the range 12–196 h. 12,13 The higher processing temperatures and longer heating times associated with conventional procedures lead to loss of components from the reaction mixture because of the volatile nature of many sodium- or phosphorus-based compounds, and it often results in nonhomogeneous products with ZrP 2 O 7 or ZrO 2 as a second phase (the segregation of these unwanted phases in the grain boundary leads to deleterious blocking effect on charge transport). 14 Therefore, the preparation temperatures must be high enough to obtain total combination of reactants and simultaneously low enough to avoid the volatilization of sodium- and phosphorus- based compounds. Further, in most conventional methods of NZP preparation, several intermittent grinding operations are necessary to obtain phase-pure compounds. Also, the conventional solid- state sintering procedures available for NZP compounds usually result in 85% of theoretical density after firing at 1200 o –1400°C for 24 –72 h (simultaneous application of pressure and temperature has also been used in some cases to achieve higher densification). This necessitates the need to explore efficient alternative methods of synthesis and sintering of NZP compounds. Recently, microwave energy has been widely used for the processing of inorganic materials and ceramics. 15 The microwave method offers several advantages over conventional methods, the foremost of which are the very short time required for the reaction and sintering, and the selectivity in energy transfer from the microwave field. 16,17 Reaction yields and structural uniformity of products had been reported to be better than in traditional meth- ods. 18 –22 However, a critical requirement in using microwaves is the coupling of at least one of the reactants to the microwave field to initiate and drive the microwave-assisted reactions. 23,24 In the present work, we report the use of highly microwave absorptive sodium di-hydrogen phosphate monohydrate (NHPM), NaH 2 PO 4 H 2 O, to synthesize NZP compositions in a single step very rapidly. NHPM can be heated to 900°C when exposed to microwaves at 2.45 GHz. Interestingly, the combination of micro- waves and nonstoichiometric oxide precursors (such as TiO 2x and partially stabilized zirconia in place of regular zirconia) for the preparation of NZP compounds has been found to enhance the reaction kinetics even further. A microwave-assisted procedure for the sintering of NZP compounds has also been demonstrated, and 97% densification has been achieved in just 30 min at sintering temperatures much lower than required in a conventional process. Regarding microwave sintering of NZP materials, there is only one earlier report available wherein Fang et al. 25 have used the microwave method to sinter Ca 0.5 Sr 0.5 Zr 4 P 6 O 24 (CSZP). Their results indicate that up to 94% density can be obtained in microwave at 1300°C using sol– gel prepared precursor powder of CSZP. II. Experimental Procedures A modified commercial microwave system (Radarange Model RC/14SE, Amana) operating at 2.45 GHz with a maximum power output of 2 kW was used for this study. Less than 60% of the total microwave power was used in all the experiments. The microwave absorption of NaH 2 PO 4 H 2 O was studied by exposing a 10 g of the salt powder (placed in a silica crucible) to microwaves. The sample temperature was measured using both platinum shielded Pt/(Pt– 10% Rh) thermocouple (at T 600°C) and dual wavelength pyrometer (for T 600°C) as described elsewhere. 18,19,26 Appro- priate amounts of powders of starting materials (batch of 10 –20 gms) such as NHPM, TiO 2 , SnO 2 ,P 2 O 5 , Fe 2 O 3 (99.9% purity; Fisher Scientific, Springfield, NJ), monoclinic-ZrO 2 , and partially yttria (5 mol%) stabilized zirconia (Zirconia Sales, GA) were used. The nonstoichiometric TiO 2x (where x is in the order of 0.01) used as a precursor (for sodium titanium phosphate compound, NTP) was prepared by reducing TiO 2 at 1100°C for 4 h in forming gas (95% N 2 and 5% H 2 ). 18 Microwave coupling characteristics of regular TiO 2 and reduced TiO 2x were also examined in a similar manner as that of NHPM. A. Bandyopadhyay—contributing editor Manuscript No. 187050. Received April 4, 2002; approved January 14, 2003. Some of the results were presented by the lead author at the 101st Annual Meeting of American Ceramic Society held at Indianapolis, IN. *Member, American Ceramic Society. Presently at Institute of Polymer Technology and Materials Engineering, Lough- borough University, Leicestershire, LE11 3TU, UK. Author to whom correspondence should be addressed. J. Am. Ceram. Soc., 87 [5] 834 – 839 (2004) 834 journal

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Page 1: Microwave-Assisted Synthesis and Sintering of NZP Compounds

Microwave-Assisted Synthesis and Sintering of NZP Compounds

Balasubramaniam Vaidhyanathan,† Dinesh K. Agrawal,*,‡ and Rustum Roy

Microwave Processing and Engineering Center, Materials Research Institute, The Pennsylvania State University,University Park, Pennsylvania 16802

We have identified the high microwave susceptibility of thesodium di-hydrogen phosphate monohydrate (NHPM),NH2PO4�H2O. This acid phosphate of sodium can be heated to>900°C when exposed to 2.45 GHz microwave radiation.Using NHPM and microwave energy, a novel single-stepsynthesis of many important crystalline NZP compounds hasbeen accomplished in a very short time. Interestingly, thecombination of microwaves and nonstoichiometric oxide pre-cursors for the preparation of NZP materials is found toreduce the reaction temperatures and enhance the reactionkinetics further. The microwave synthesis method is found tobe simple and fast, minimizing the loss of volatile chemicalspecies from the reaction. A microwave-assisted procedure forthe rapid sintering of NZP compounds has also been demon-strated. Densification (>97%) has been achieved in <30 min atsintering temperature much lower than normally required inconventional processes. Finer microstructure and better den-sification are the main advantages of the microwave sinteringin this study.

I. Introduction

APARENT compound of a very large family of important andinteresting inorganic phosphates, NaZr2(PO4)3, commonly

known as NZP,1,2 has a special open framework structure3 withcorner-linked ZrO6 octahedra and PO4 tetrahedra.4 The covalentnetwork skeleton NZP is remarkably stable toward temperatureand chemical substitution.5,6 Compounds based on NZP structurehave found application as superionic conductors,5 candidates forceramic nuclear waste form,2,7 catalysts for NOx reduction, and aresuitable for low thermal expansion applications.8 The conventionalsolid-state synthesis methods presently available for the prepara-tion of NZP compounds are complex and time-consuming.9–11

Typical solid-state synthesis method employs a high calcinationtemperature of 1100o–1300°C and long soaking times in the range12–196 h.12,13 The higher processing temperatures and longerheating times associated with conventional procedures lead to lossof components from the reaction mixture because of the volatilenature of many sodium- or phosphorus-based compounds, and itoften results in nonhomogeneous products with ZrP2O7 or ZrO2 asa second phase (the segregation of these unwanted phases in thegrain boundary leads to deleterious blocking effect on chargetransport).14 Therefore, the preparation temperatures must be highenough to obtain total combination of reactants and simultaneouslylow enough to avoid the volatilization of sodium- and phosphorus-based compounds. Further, in most conventional methods of NZP

preparation, several intermittent grinding operations are necessaryto obtain phase-pure compounds. Also, the conventional solid-state sintering procedures available for NZP compounds usuallyresult in �85% of theoretical density after firing at 1200o–1400°Cfor 24–72 h (simultaneous application of pressure and temperaturehas also been used in some cases to achieve higher densification).This necessitates the need to explore efficient alternative methodsof synthesis and sintering of NZP compounds.

Recently, microwave energy has been widely used for theprocessing of inorganic materials and ceramics.15 The microwavemethod offers several advantages over conventional methods, theforemost of which are the very short time required for the reactionand sintering, and the selectivity in energy transfer from themicrowave field.16,17 Reaction yields and structural uniformity ofproducts had been reported to be better than in traditional meth-ods.18–22 However, a critical requirement in using microwaves isthe coupling of at least one of the reactants to the microwave fieldto initiate and drive the microwave-assisted reactions.23,24 In thepresent work, we report the use of highly microwave absorptivesodium di-hydrogen phosphate monohydrate (NHPM),NaH2PO4�H2O, to synthesize NZP compositions in a single stepvery rapidly. NHPM can be heated to �900°C when exposed tomicrowaves at 2.45 GHz. Interestingly, the combination of micro-waves and nonstoichiometric oxide precursors (such as TiO2�x

and partially stabilized zirconia in place of regular zirconia) for thepreparation of NZP compounds has been found to enhance thereaction kinetics even further. A microwave-assisted procedure forthe sintering of NZP compounds has also been demonstrated, and�97% densification has been achieved in just 30 min at sinteringtemperatures much lower than required in a conventional process.Regarding microwave sintering of NZP materials, there is only oneearlier report available wherein Fang et al.25 have used themicrowave method to sinter Ca0.5Sr0.5Zr4P6O24 (CSZP). Theirresults indicate that up to 94% density can be obtained inmicrowave at 1300°C using sol–gel prepared precursor powder ofCSZP.

II. Experimental Procedures

A modified commercial microwave system (Radarange ModelRC/14SE, Amana) operating at 2.45 GHz with a maximum poweroutput of 2 kW was used for this study. Less than 60% of the totalmicrowave power was used in all the experiments. The microwaveabsorption of NaH2PO4�H2O was studied by exposing a 10 g of thesalt powder (placed in a silica crucible) to microwaves. The sampletemperature was measured using both platinum shielded Pt/(Pt–10% Rh) thermocouple (at T � 600°C) and dual wavelengthpyrometer (for T � 600°C) as described elsewhere.18,19,26 Appro-priate amounts of powders of starting materials (batch of 10–20gms) such as NHPM, TiO2, SnO2, P2O5, Fe2O3 (99.9% purity;Fisher Scientific, Springfield, NJ), monoclinic-ZrO2, and partiallyyttria (5 mol%) stabilized zirconia (Zirconia Sales, GA) were used.The nonstoichiometric TiO2�x (where x is in the order of 0.01)used as a precursor (for sodium titanium phosphate compound,NTP) was prepared by reducing TiO2 at 1100°C for 4 h in forminggas (95% N2 and 5% H2).18 Microwave coupling characteristics ofregular TiO2 and reduced TiO2�x were also examined in a similarmanner as that of NHPM.

A. Bandyopadhyay—contributing editor

Manuscript No. 187050. Received April 4, 2002; approved January 14, 2003.Some of the results were presented by the lead author at the 101st Annual Meeting

of American Ceramic Society held at Indianapolis, IN.*Member, American Ceramic Society.†Presently at Institute of Polymer Technology and Materials Engineering, Lough-

borough University, Leicestershire, LE11 3TU, UK.‡Author to whom correspondence should be addressed.

J. Am. Ceram. Soc., 87 [5] 834–839 (2004)

834

journal

Page 2: Microwave-Assisted Synthesis and Sintering of NZP Compounds

For the microwave-assisted synthesis of NZP compounds, thestoichiometric amounts of oxide precursors were thoroughlymixed with NHPM in an agate mortar and placed inside themicrowave cavity in silica crucible. Appropriate thermal insulation(Fiberfrax Duroboard) was used to minimize the heat losses. Thereaction temperatures were varied from 600°–900°C and thesoaking times from 10 to 30 min. Conventional solid-state synthe-sis of the parent sodium zirconium phosphate, NaZr2(PO4)3, wasalso conducted for comparison. All the synthesized samples wereground using an agate pestle and mortar and the phase-composition was determined by powder XRD (Scintag, SantaClara, CA). The quality of the microwave prepared NZP powderwas also examined by infrared spectroscopy (Magna-IR-560,Nicolet Instruments, Madison, WI; the measurements were con-ducted on thin specimen disks made using a mixture of KBr andthe NZP sample powder).

For microwave sintering, 3 wt% of an acryloid resin binderwas added to the as-prepared NZP powder before compaction.This binder was chosen because of its excellent performance inproviding homogeneous mixture and high green density of thesample. There was no microwave absorption measurementperformed for the binder. No sintering aid was used. Circularpellets of 1.25-cm diameter were made using a uniaxial labo-ratory press (Fred S. Carver, Wis.). The green densities of thecold-compacted pellets ranged between 57% and 68% theoret-ical. The NZP pellets were sintered in air in the microwavechamber described above. MoSi2 rods were used as secondarysusceptors in the microwave field. The sintering was conductedat various temperatures from 1000°–1300°C and the soakingtime varied between 20 – 60 min. Conventional solid-statesintering of NZP compound was also conducted using an electricfurnace for comparison. The sintered samples were characterizedfor density by Archimedes method with xylene as the liquidmedium; Sartorius balance with an accuracy of 10�4 g was usedfor measuring sample weight; three sample pellets were taken forevery set of conditions and the average value is reported. Micro-structures were examined by scanning electron microscopy (ModelS-3500N, Hitachi, Tokyo, Japan) integrated with an X-ray micro-analysis system.

III. Results and Discussion

Figure 1 depicts the microwave heating characteristics of theNHPM sample. The time-temperature profile shows an initialrapid increase in temperature and a final leveling-off region. Byvisual examination, dehydration in NaH2PO4�H2O appeared totake place in two stages. The first step apparently starts within5 min of exposure (appearance of bubbles) and is completedwithin 7 min (when bubbling stops temporarily). This maycorrespond to the removal of one H2O molecule, water ofcrystallization. The second stage begins at �7 min and lasts upto 9 min. This stage corresponds to the removal of theremaining one (chemically bound) H2O molecule from thestructure resulting from a condensation reaction. The tempera-tures corresponding to the removal of these water molecules aremarked by arrows in the figure as 243° and 391°C, respectively.The weight-loss measurements were conducted after 7 and 9min, which clearly confirm the removal of water molecules atthese temperatures. The two stages of weight losses can becorrelated to the temperatures of decomposition of watermolecules by corroborative TGA/DTA measurements (see Fig.2(a) and (b)). The TGA results confirm the two-step dehydra-tion procedure at temperatures 218° and 352°C. Incidence ofthese transitions is confirmed further by DTA thermograms.The melting point of dehydrated product is found to be 612°C.The slightly higher estimates of these temperatures during themicrowave procedure could be a manifestation of the very rapidheating rates involved. Thus, the microwave absorption inNHPM continues to occur even after two stages of dehydration,and resulting into the further heating of anhydrous NaPO3 tohigher temperatures. The material melts and turns red hot.Thereafter, the microwave coupling and thermal losses competeto maintain a constant temperature. Thus the different stages

Fig. 1. Microwave heating characteristics of NHPM. Line is drawn as aguide to the eye.

Fig. 2. Thermal analysis results on NHPM: (a) TGA and (b) DTAthermograms (heating rate used is 10°C/min and the measurements wereperformed in air).

May 2004 Microwave-Assisted Synthesis and Sintering of NZP Compounds 835

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involved in the microwave heating of NHPM can be depicted asthe following:

(i) NaH2PO4�H2O (crystalline) 3 NaH2PO4 � H2O(ii) NaH2PO4 3 Hot NaPO3 � H2O(iii) NaPO3 3 Molten NaPO3

The dielectric loss characteristics of the resulting product ineach stage of the microwave heating of NHPM is sufficient enoughto provide sustained microwave absorption, leading to furthertemperature escalation. However, this high microwave absorptionof NHPM can be contrasted with other sodium salts, such asNa2HPO4�2H2O and Na3PO4�12H2O, wherein the microwave cou-pling ceases after the slight initial increase in temperature (up to amaximum of 200°C) and the samples do not heat up further. Theseexperiments were conducted for up to 15 min of microwaveexposure. It is possible that the replacement of H� ion by Na� ion(in a chemical sense) in the salt structure (of Na2HPO4�2H2O)leads to a geometrically tightly packed structure, thereby enhanc-ing the barrier to the rotation of H2O molecules (it is suggested thatthe microwave absorption in hydrated salts could be due to theenergy transfer from microwave field to the rotational modes).15

Such barrier-increasing effects are known in solution chemistrywhere the effect of ionic substitutions on rotational barriers hasbeen studied.27

Using the high microwave absorbing capability of NHPM, anumber of technologically important NZP compounds were syn-thesized in rapid time scales. Figure 3 gives the X-ray diffracto-grams of microwave-prepared parent NZP composition using

appropriate amounts of NHPM, m-ZrO2, and P2O5 at varioustemperatures for a soaking period of 20 min. It is evident that,although the formation of NZP starts at �600°C, some amount ofresidual zirconium phosphate (ZP) phase exists up to 800°C.However, the microwave treatment at 900°C for 20 min resulted inphase pure NZP compound with good crystallinity (Fig. 4). Thecalculated lattice parameters (a � 8.802 Å and c � 22.756 Å) ofthe microwave prepared NZP were found to match well with thereported data.28 It is interesting to note that this is a single-stepsynthesis without involving any intermittent grinding procedures.The uniform, volumetric heating in microwave, and the bulknucleation might have resulted in the rapid formation of phase-pure NZP. Also in the microwave-assisted process, because thecrucible is not heated directly and only the reactants couple tomicrowaves, contamination by the crucible material is virtuallyeliminated.29,30

Figure 5 provides the XRD patterns of microwave prepared andconventionally synthesized NZP compound using the same reac-tants. As can be seen the conventional product retains a largeamount (�80%) of ZP phase, even after 1 h soaking. It took 24 hof total cycle time at 1000°C and two intermittent grindingoperations to produce a single-phase NZP in conventional process.The lower soaking temperature and shorter processing time of themicrowave technique avoid the undesirable phosphate losses fromthe reaction mixture. This has been confirmed by product weight-loss measurements. It also results in a homogeneous and single-phase NZP compound. The microwave-prepared NZP compoundis further characterized using infrared spectroscopy (Fig. 6). TheIR spectrum is dominated by the vibrational bands of the phos-phate units and indicates strong vibrational bands at approximately1203, 1038, 643, 575, 555, and 420 cm�1, all of which match wellwith reports in the literature.28,31

Fig. 3. X-ray diffractograms of microwave-prepared NZP at varioustemperatures.

Fig. 4. Powder X-ray diffraction pattern of the microwave-synthesizedNaZr2(PO4)3 at 900°C using monoclinic zirconia precursor.

Fig. 5. XRD patterns of the microwave-prepared and conventionallysynthesized NZP compounds using the same reactants.

Fig. 6. Infrared spectrum of the microwave-prepared NZP sample.

836 Journal of the American Ceramic Society—Vaidhyanatha et al. Vol. 87, No. 5

Page 4: Microwave-Assisted Synthesis and Sintering of NZP Compounds

In a similar procedure, single-step microwave-assisted synthesisof NaSn2(PO4)3 (NSP) has also been accomplished using NHPM,SnO2, and P2O5 as precursors. Complete formation of NSP isachieved in just 12 min at 800°C. Powder X-ray diffractionpatterns (Fig. 7) indicate a well-crystallized NaSn2(PO4)3.

It was envisaged that the use of nonstoichiometric precursors,such as partially stabilized zirconia (with 5 mol% Y2O3) in placeof regular zirconia, may further enhance the reaction kinetics of theformation of NZP compounds because of its better microwaveabsorption characteristics.18 Figure 8 provides the X-ray diffrac-togram of the microwave prepared NZP sample using partiallystabilized zirconia (PSZ) as one of the starting materials (alongwith NHPM and P2O5). Indeed, the single-phase parentNaZr2(PO4)3 has been formed in �15 min at 750°C in this case(whereas a soaking temperature of 900°C was required whenregular zirconia is used). The good crystallinity and high-phasepurity is evident from the XRD pattern. These results correlatewell with the earlier reports by Vaidhyanathan et al.18 on themultifold enhancement in the reaction kinetics (with an associatedreduction in synthesis temperature) when PSZ was used as aprecursor for the processing of PZT compositions. It is alsointeresting to note that a slight reduction in processing temperaturealso is observed while using PSZ during the conventional synthesisof NZP, though the procedure requires more than 20 h ofheating.32 In addition, the use of PSZ for NZP synthesis is alsofound beneficial in increasing the electrical conductivity of theseNa� ion conductors.32 The combined use of microwave irradiation

and nonstoichiometric PSZ precursor may have also resulted inaltered reaction pathways for the synthesis of NZP. The rapid-reaction rates associated with the microwave synthesis of NZPmaterials do not rule out the possibility of nonthermal mecha-nisms33 being operative.

Microwave synthesis of another important compound in theNZP family, namely sodium titanium phosphate (NTP), was alsoprepared using NHPM with either anatase-TiO2 or TiO2�x asprecursor materials. Well-crystallized NTP was formed in bothcases in �15 min of soaking at 800°C (Fig. 9). However, it wasfound that the heating rate was higher when using nonstoichio-metric titania, the reaction took less time for completion, andsingle-phase NTP was obtained even at 700°C after 15 min ofprocessing. This can be correlated to the differences in themicrowave absorption of TiO2 and TiO2�x. The nonstoichiometrictitania not only absorbs microwaves more efficiently, resulting inhigher heating rates, but it also attains much higher temperatures(1000°C) than stoichiometric TiO2 (800°C) when exposed tomicrowaves for the same duration (Fig. 10).18 The high-frequencydielectric measurements performed on anatase-TiO2 and defectiverutile TiO2�x as a function of temperature indicated a substantialincrease in the dielectric loss of TiO2�x due to the large number ofdefects (oxygen vacancies) present.34 It is worth mentioning herethat similar enhancement in reaction kinetics and lowering of thereaction temperatures has been observed earlier while using

Fig. 7. Powder X-ray diffractogram of the microwave-prepared sodiumtin phosphate compound.

Fig. 8. XRD pattern of the microwave-synthesized NZP sample at 750°Cfor 12 min using PSZ as the precursor.

Fig. 9. X-ray diffractograms of microwave-prepared Sodium TitaniumPhosphate (NTP) using (a) regular TiO2 and (b) reduced TiO2�x

precursors.

Fig. 10. Time-temperature profiles for the microwave heating of TiO2 (inair) and TiO2�x (in nitrogen atmosphere).

May 2004 Microwave-Assisted Synthesis and Sintering of NZP Compounds 837

Page 5: Microwave-Assisted Synthesis and Sintering of NZP Compounds

Ta2O5�x for the microwave processing of high-frequency dielec-trics such as barium magnesium tantalates.35

The microwave process had also been found to be efficient forthe sintering of NZP materials. Figure 11 provides the results ofthe microwave sintering of NZP pellets of NaZr2(PO4)3. Thesamples have been microwave sintered between 1000°–1300°C for30 min and �97% densification has been achieved at 1300°C. The

sintered density was found to increase with increasing temperatureas well as with soaking time. However, corresponding conven-tional sintering experiments (for a soaking period of 2 h) alwaysresulted in lower sintered densities (refer Fig. 11) at any giventemperature, and the densification tends to level off at highertemperatures. For example, it took almost 8 h of soaking inconventional sintering to achieve 95% density in NZP at 1200°C,whereas in microwave, a 30-min soaking at the sintering temper-ature resulted in similar densification. The microwave sinteringalso resulted in smaller average grain size compared with conven-tional methods. Figure 12 provides the SEM photographs of themicrowave and conventionally sintered NZP samples at 1100°C.The overall high densification and extensive “necking” (indicatingthe possibility of enhanced diffusion—material flow—at the inter-facial region) in the microwave product is evident. Figure 13shows the scanning electron micrographs of the microwave sin-tered NZP for different soaking periods (viz., for 30 and 60 min at1200°C). Better densification and relatively larger average grainsize resulted from longer sintering schedules. The effect of greendensity on the microwave sintering behavior of the NZP com-pound is depicted in Fig. 14. Final densification achieved wasfound to increase with increasing green density. This could bebecause of better particle-particle contact under higher compac-tion, enabling facile material transport and diffusion for thesintering to occur. The electric field component of the microwaveradiation may also play a definite role36 in enhancing the NZPdensification as suggested in the case of other complex oxidematerials such as titanates37 and tantalates.35

Fig. 11. Densification results of the microwave and conventional sinter-ing of NZP.

Fig. 12. Microstructures of the (a) microwave and (b) conventionallysintered NZP samples at 1100°C.

Fig. 13. SEM photographs of the NZP specimen microwave sintered at1200°C for (a) 30 min and (b) 60 min.

838 Journal of the American Ceramic Society—Vaidhyanatha et al. Vol. 87, No. 5

Page 6: Microwave-Assisted Synthesis and Sintering of NZP Compounds

IV. Conclusions

A novel microwave-assisted single-step procedure for the syn-thesis of a number of important members of NZP family in �20min has been demonstrated. This was made possible by identifyingthe high microwave absorption of sodium di-hydrogen phosphatemonohydrate (NHPM) and using this as one of the reactants. Themicrowave products exhibited high-phase purity and good crystal-linity. The microwave-assisted synthesis method is found to besimple, fast, minimizing the loss of volatile species. The method isquite general and can be applied to other systems, such as anumber of technologically important “pigment” compositionsbased on NZP with a finer microstructure has been accomplishedin rapid time scales, and the results were presented elsewhere.38

Interestingly, the combination of microwaves and nonstoichiomet-ric precursors (such as TiO2�x and partially stabilized zirconia inplace of regular zirconia) for the preparation of NZP is found toreduce the synthesis temperatures and enhance the reaction rateseven further. A microwave-assisted procedure for the rapid sinter-ing of NZP materials has also been demonstrated and �97%densification has been achieved at temperatures lower than re-quired for conventional process. The effect of using different typesof PSZ stabilized with Y2O3, CeO2, MgO, and CaO (also the effectof varying the amounts of individual additives in PSZ) for themicrowave-assisted processing of NZP compounds will be inter-esting to study.

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Fig. 14. Effect of green density on the microwave sintering behavior ofNZP. Line is to guide the eye.

May 2004 Microwave-Assisted Synthesis and Sintering of NZP Compounds 839